The present invention relates to a modulation technique which transmits multiple a high data rate signals through a band limited channel.
It is always desirable to provide data at higher data rates through channels which have limited bandwidth. Many modulation techniques have been developed for increasing the data rate through a channel. For example, M-ary phase shift keyed (PSK) and Quadrature Amplitude Modulation (QAM) techniques permit compression by encoding a plurality of data bits in each transmitted symbol. Such systems have constraints associated with them. First, the hardware associated with such systems is expensive. This is because these techniques require a high level of channel linearity in order to operate properly. Consequently, extensive signal processing must be performed for carrier tracking, symbol recovery, interpolation and signal shaping. Second, such techniques are sensitive to multipath effects. These effects need to be compensated for in the receiver. Third, these systems often require bandwidths beyond those available in some applications (for example in-band on-channel broadcast FM subcarier service) for the desired data rates.
It is also desirable to provide for several data signals through a channel. Some modulation techniques utilize the channel completely, while others leave some aspect of the channel unused. Frequency domain multiplexing and time domain multiplexing are two techniques for sharing a channel among a plurality of signals. By sharing the channel in this manner, the overall throughput through the channel is increased.
In accordance with principles of the present invention, a digital data modulator includes a source of a plurality of digital data signals having a common data bit period. A plurality of encoders each encode a corresponding one of the plurality of digital data signals using a variable pulse width code having edges occurring in respective non-overlapping intervals within the data bit period. A plurality of pulse signal generators each generate respective pulses representing the edges of the corresponding one of the encoded plurality of digital data signals. A carrier signal generator generates a carrier signal having carrier pulses corresponding to the respective pulses.
In accordance with another aspect of the present invention, a digital data demodulator includes a source of a modulated signal including successive bit periods, each bit period having a plurality of non-overlapping intervals respectively associated with corresponding intervals in successive bit periods, each interval containing a carrier pulse, spaced relative to carrier pulses in other associated intervals, representing a corresponding variable pulse width encoded digital data signal. A detector demodulates the modulated signal to generate pulses corresponding to received carrier pulses. A plurality of decoders each decode pulses received in a respective one of the plurality of associated intervals in the bit period to generate the corresponding digital data signal.
The technique according to the principles of the present invention provides for simultaneous transmission of a plurality of independent high data rate signal through a single channel. A system according to the present invention may be implemented using relatively inexpensive circuitry, is insensitive to multipath interference, and provides substantial bandwidth compression.